Well, judging from all the comments they've been getting, LJ's users are pretty overwhelmingly against the idea.
So, how do we get SixApart to stop this selling of LiveJournal's users to the highest bidder? Easy. We boycott their advertisers.
Their first sponsored community is for "The Science of Sleep" ( scienceofsleep ), which is distributed by Warner, so I suggest that for starters, we boycott Warner, until they decide to pull the community or until SixApart changes their policies regarding sponsored communities... whichever happens first.
Basically, these new communities will be specially featured on the main LJ page, everytime you visit the site. They will also get preferential treatment on search results / interest searches, which will tend to decrease visitors and new friends for ordinary users. Perhaps more disconcertingly, the widespread use of sponsored journals as marketing tools will increase the potential for ordinary users to be the target of corporate takedown notices and LiveJournal suspensions of people who feature icons, content, polls, or communities that potentially violate the copyrights of these sponsors, or that independently compete for the same audience that these corporate sponsors are paying for.
I'm absolutely not saying that it's a bad thing to have artists using LiveJournal. Musicians like <lj user="imomus"> have been using LiveJournal for years. What I am saying, however, is that there are real risks to giving any of them preferential treatment, and to commoditizing the eyeballs of LJ's users. The experience will fundamentally change as a result, to the detriment of people who have paid good money to use the service.
So, if you don't want this new LiveJournal "feature", simply let people know in your journal, and maybe leave a message for the advertiser on the community, telling them politely that as much as you may want to see the movie or support their products, you cannot under the circumstances. Also, if you see another Sponsored Community, let me know, so I can add the name of the sponsor to my journal.
I would also encourage anyone who likes this idea to take the ball and run with it. Create an LJ community for it, and I will gladly send people your way.
Threaten a boycott and encourage your friends to do so, and I promise you, this policy will not last, period. SixApart purchased LiveJournal with the full knowledge of it's promise to stay advertisement free. They have broken and bent that promise badly, and it is time for us to remind them that there are reasonable limits to our tolerance for advertising in LiveJournal.
We will not be sold.
*** UPDATE! --
A user has created antisponsoredlj , which will be used to organize a boycott against advertisers of sponsored communities.
An LJ user just finished creating a community protesting scienceofsleep's use of "bad guerilla marketing" -- be sure to check out the new community scienceofsuck.
Apple-garden has also designed some icons opposed to sponsored communities, for those interested.